<span>

  <metal:block define-macro="test">
    <div metal:define-slot="content">no content provided</div>
  </metal:block>

  <div metal:use-macro="test">
    <div metal:fill-slot="content" tal:condition="fillit">my content</div>
  </div>

  <div metal:use-macro="test">
    <div metal:fill-slot="content" tal:condition="not:fillit">my content</div>
  </div>

</span>
